Directly modulating the analog intensity of a semiconductor laser with an electrical signal is considered among the simplest methods known in the art for transmitting optical signals carrying information, such as sound and video signals, through optical fibers. Such optical signals, however, are susceptible to noise and distortion.
Much of the noise which degrades signal quality is produced by light scattering and reflection effects within the optical fiber. Rayleigh scattering occurs due to localized variations in density within optical fibers, which are inherent to glass. Reflection occurs due to discrete components, such as, for example, connectors, splices, and detectors.
Both scattering and reflection contribute to noise generated at the source and noise received at the end of the system. Optical feedback due to discrete reflectors or distributed reflection from Rayleigh backscattering in the glass fiber can cause instabilities in the light source, including optical power fluctuations and distortion instabilities. Such instabilities degrade the quality of the transmitted analog signal. Furthermore, if a portion of the signal is twice reflected due to discrete reflectors, Rayleigh backscattering, or a combination of the two, interferometric noise and distortion will be produced at the receiving end of the link from the mixing in the photodetector of directly transmitted light with light that has been twice reflected. These effects are particularly detrimental to systems such as satellite communications, CATV, and mobile telecommunications networks which require a high signal-to-noise ratio and a high degree of linearity for multi-channel transmission.
Due to the fundamental properties of glass fibers, Rayleigh scattering is a largely unavoidable effect. Noise due to discrete reflectors can be minimized by using high-quality components, but this is not always practical. In many cases, systems must work on previously installed fiber links which include high-reflection components.
The present invention, accordingly, is addressed to a low-cost apparatus and method for suppressing noise and distortion due to scattering and reflections.